1. Field of the Invention
The present invention relates to a method of determining deterioration of pressurizing performance of a spot welding gun which sandwiches a work piece between a pair of electrode tips, that is, a movable side electrode tip and an opposition side electrode tip, and, while applying pressurizing force to the work piece, performs welding of the work piece.
2. Description of Related Art
In spot welding that uses a spot welding gun, welding is performed on a work piece while a specified pressurizing force is being applied to the work piece sandwiched between a pair of opposing electrode tips, that is, a movable side electrode tip and an opposition side electrode tip of the welding gun. The opposing separation of the pair of electrode tips is set to a predetermined amount by directly moving the movable side electrode tip relative to the fixed side electrode tip having been positioned at a prescribed position.
In general, the movable side electrode tip is controlled so as to be moved by the driving force of a servo motor in a direction of closing or opening the opposing separation between a pair of electrode tips at a specified speed to a predetermined position. The work piece is thereby subjected to a specified pressurizing force between the pair of electrode tips so that stable quality of welding is maintained.
When a spot welding system is to be set up anew (at the time of installation) or processing conditions for welding are to be altered, a parameter adjusting operation is performed in order to decide operating parameters of the mounted welding gun taking into account dynamical characteristics of the welding gun such as rigidity or friction. Next, a calibration adjusting operation is performed for calibration of a torque command provided to the servo motor that drives the spot welding gun and an actual pressing force produced at the distal end of the electrode tip by the torque command. These adjusting operations are preliminary operations to be performed before welding is carried out with the spot welding gun. Then, welding operation is performed repeatedly with the spot welding gun.
However, it is known that a driving mechanism comprising a servo motor or a reduction gear necessarily deteriorates due to secular change such as wear of a sliding mechanism. Thus, machine elements such as ball screws and bearings are subjected to secular change due to faults or wear after usage for a long term. If the secular change is large, for example, if the welding gun can no longer operate, even if the robot controller outputs torque command to the servo motor, the driving mechanism of the welding gun itself cannot be operated, so that deviation of the pulse encoder value from the torque command is produced, leading to output of an alarm from the robot controller. An operator can thus recognize the occurrence of an anomaly in the spot welding robot. On the other hand, if the magnitude of secular change is small, for example, when a slight wear or the like is produced in a sliding mechanism, the magnitude of torque used in actual pressing of the work piece may be decreased due to the increase of the wear in the mechanism, but does not give rise to output of an alarm. Thus, only the pressurizing force produced by the welding gun is decreased. In such a situation, it is necessary to perform diagnosis of the welding robot such as measurement of the pressurizing force in a predetermined timing, and to perform adjusting operation again for parameters and calibration in order to prevent the quality of welding from being lowered. However, since a pressure sensor used for measuring the pressurizing force is expensive, adequate number of sensors may not be provided, and then, welding may be performed without recognizing the change of pressurizing force, which may lead to degradation of welding quality.
Therefore, in order to avoid degradation of welding quality, a deterioration diagnostic method has been proposed for diagnosing deterioration of a drive mechanism for a spot welding gun which uses a servo motor as a driving source.
Various methods have been known for diagnosing deterioration of a drive mechanism, and an example of deterioration diagnosing method applied to a spot welding gun is disclosed in Japanese Patent Publication No. 2007-29994. In the invention disclosed in Japanese Patent publication No. 2007-29994, the pressurizing time from the start of driving the electrode (movable side electrode) until the work piece is sandwiched under a predetermined pressing force and the pressurizing position (pressurized position) of the electrode when the work piece is sandwiched under a predetermined pressing force, are obtained, and deterioration of the drive mechanism due to wear of the mechanism can be diagnosed by analyzing the obtained pressurizing time and pressurizing position using a method of statistical analysis.
However, with respect to obtaining pressurizing position as disclosed in Japanese Patent Publication No. 2007-29994, it is necessary that the wear of the distal end of the electrode tip has been accurately measured. Usually, the distal end of the electrode tip is deformed or worn for each welding, but the amount of wear of the distal end of the electrode tip is measured only once after several cycles of operation. Thus, the obtained pressurizing position of the electrode includes errors due to the wear of the distal end of the electrode tip, and the deterioration of the drive mechanism due to wear of the mechanism may not be diagnosed accurately. There is also a problem that, since the rise-up behavior of the pressurizing force is influenced by individual difference of the work pieces and teaching deviation, it is difficult to grasp the deterioration of the drive mechanism accurately.